Vehicles today use, for example, urea as reductant in SCR (selective catalytic reduction) systems which comprise an SCR catalyst with which said reductant and NOx gas can react and be converted to nitrogen gas and water. Various types of reductants may be used in SCR systems. AdBlue is an example of a commonly used reductant.
One type of SCR system comprises a container which holds a reductant. The SCR system also has a pump adapted to drawing said reductant from the container via a suction hose and to supplying it via a pressure hose to a dosing unit situated adjacent to an exhaust system of the vehicle, e.g. adjacent to an exhaust pipe of the exhaust system. The dosing unit is adapted to injecting a necessary amount of reductant into the exhaust pipe upstream of the SCR catalyst according to operating routines which are stored in a control unit of the vehicle. To make it easier to regulate the pressure when there are small or no dosing amounts, the system comprises also a return hose which runs back to the container from a pressure side of the system. This configuration makes it possible to cool the dosing unit by means of the reductant which, during cooling, flows from the container via the pump and the dosing unit and back to the container. The dosing unit is thus provided with active cooling. The return flow from the dosing unit to the container may be substantially constant and is not controlled or regulated by means of appropriate valves or such units.
As the dosing unit is situated adjacent to the vehicle's exhaust system which becomes warm during operation of the vehicle, e.g. depending on the load, there is risk of the dosing valve becoming overheated. Overheating of the dosing unit may entail degradation of its functionality, potentially impairing its performance.
The dosing unit comprises electrical components, certain of them being provided with a circuit card. Said circuit card may for example be adapted to controlling the dosing of AdBlue to the vehicle's exhaust system. For various reasons, these electrical components are sensitive to high temperatures. Too high temperatures of the dosing unit may result in degradation of the electrical components, potentially leading to expensive repairs at a service workshop. Moreover, the reductant present in the dosing unit may at least partly crystallise at too high temperatures, potentially leading to obstruction of the dosing unit. It is therefore of the utmost importance that the temperature of the dosing unit of the SCR system should not exceed a critical level.
Cooling the dosing unit of a vehicle's SCR system currently takes place continuously during the vehicle's ordinary operation as a result of the reductant circulating within the SCR system as indicated above. Cooling the dosing unit during operation of the vehicle currently works satisfactorily.
After operation of the vehicle a large amount of thermal energy caused by its operation is stored in primarily the exhaust system. This thermal energy may be led to the dosing unit from, for example, a silencer and the SCR catalyst and may warm the dosing unit to a temperature which exceeds a critical value.
When the vehicle is switched off and the exhaust flow in the exhaust system consequently ceases, the reductant dosing unit is cooled for a predetermined time, e.g. about 30 minutes, by said reductant in the same way as during ordinary operation.
This arrangement entails certain disadvantages. One is a relatively large amount of energy used to power the pump in the SCR system after the vehicle has been switched off. Any vehicle battery used to power the pump of the SCR system might thus be discharged or reach an undesirably low charge level.
Another disadvantage of the dosing unit being cooled in the same way as during ordinary operation is that the pump of the SCR system emits disturbing noise which for example a driver of the vehicle may find irritating, particularly when he/she has to sleep in the cab after a driving run or is in the immediate vicinity of the vehicle.
There is thus a need to improve current methods for cooling the dosing unit in the SCR system after the vehicle has been switched off, in order to reduce or eliminate the above disadvantages.
DE 102007000666 A1 describes a device for supply of reducing agent to an exhaust duct during catalytic exhaust cleaning and discusses cooling of an injection valve for reducing agent after the engine and consequently the exhaust flow have been switched off. The arrangement in DE 102007000666 A1 comprises a cooling jacket which for cooling purposes surrounds the injection valve and is adapted to having reducing agent flow through it.